1. Field of the Invention
The present invention relates to a heat-resistant packaging container made of a polyester resin, and particularly to a heat-resistant packaging container which is formed by stretch-blow molding a preform of a polyester resin, having a thin-walled body portion and a bottom face recessed into the container.
2. Background Art
In general, packaging containers such as bottles or wide mouthed jars made of a resin, in which a grounding edge around a bottom face is formed together with a body portion so as to have a small wall thickness by stretch-blow molding, exhibit increased durability, transparency, gas-barrier properties, heat resistance and the like because molecules therein are biaxially oriented resulting from vertical stretching by mechanical force, and lateral stretching by air blowing. However, as long as the heat resistance is concerned, since the material is a resin, there is a limitation to the heat-resistant temperature and heat deformation will occur over a certain temperature.
This heat deformation depends on the region of the container, and is likely to occur at a neck portion or a bottom portion in particular. As to the neck portion, it is possible to improve the heat resistance by post treatment such as crystallization. As to the bottom portion, a bottom structure in which a bottom face is formed to be recessed into the container is being employed to improve the resistance to heat deformation.
A temperature of heat deformation of the resin used as the material is about 75.degree. C. in the case of polyethylene terephthalate (PET), for example. Accordingly, at temperatures higher than that temperature, heat deformation occurs at the bottom portion and the levelness of the grounding edge around the bottom face is impaired to render the container unstable, so that the container, if it is of a small size, is liable to fall down, for example, due to vibrations.
In view of the above, as the materials of containers that are required to endure temperatures exceeding 75.degree. C., heat-resistant resins such as polycarbonate (PC) and polypropylene (PP) are adopted. However, PP is inferior to PET in terms of transparency and rigidity of a product, while PC has a problem that the use thereof is restricted because of the problem of oozing out environmental hormones, so that it is difficult to adopt these resins as the materials of containers for packaging foodstuffs or beverages.
In view of the above, polyethylene naphthalate (PEN), a polyester resin similar to PET, whose temperature of heat deformation is about 120.degree. C. is being adopted. However, even in the case of PEN, it is impossible to avoid the heat deformation in the bottom portion. The conventional bottom structure, when heated at a temperature of 100.degree. C. or higher for a long time, causes an irregular deformation on the grounding edge around the bottom face and the grounding edge is expanded out due to a load of contents so that the levelness thereof is impaired and the container is likely to become unstable.
The expanding deformation of the bottom face is attributed to difficulty of molding the grounding edge around the bottom face. The grounding edge around the bottom face is under the condition that the wall thickness thereof is irregularly distributed even though the degree of the irregularity is very small, so that the portions having small wall thicknesses become weak points of the grounding edge. At these thin-walled portions, expanding deformation due to the load under heating is more likely to occur than other portions, to generate unevenness on the grounding edge thereof. This is the reason why the stability of container is impaired after heating. In order to suppress the expanding deformation, many bottom structures have been developed up to now, however, it is almost impossible to eliminate the expanding deformation thoroughly by means of structural measures, and hence the stability after heating must be maintained while allowing a certain compromise.